Sinterable semi-crystalline powder and near-fully dense article formed therein

ABSTRACT

A laser-sinterable powder product has been prepared having unique properties which allow the powder to be sintered in a selective laser sintering machine to form a sintered part which is near-fully dense. For most purposes, the sintered part is indistinguishable from another part having the same dimensions made by isotropically molding the powder. In addition to being freely flowable at a temperature near its softening temperature, a useful powder is disclosed that has a two-tier distribution in which substantially no primary particles have an average diameter greater than 180 μm, provided further that the number average ratio of particles smaller than 53 μm is greater than 80%, the remaining larger particles being in the size range from 53 μm to 180 μm. A powder with slow recrystallization rates, as evidenced by non-overlapping or slightly overlapping endothermic and exothermic peaks in their differential scanning calorimetry characteristics for scan rates of on the order of 10° C. to 20° C. per minute, will also result in sintered parts that are near-fully dense, with minimal dimensional distortion.

This application is a continuation-in part of commonly assignedapplication Ser. No. 08/298,076, filed Aug. 30, 1994, now U.S. Pat. No.5,527,877 which is a continuation-in-part of commonly assignedapplication Ser. No. 07/980,004, filed Nov. 23, 1992, now U.S. Pat. No.5,342,919.

This invention is in the field of rapid prototyping, and is moreparticularly directed to materials for producing prototype parts by wayof selective laser sintering.

BACKGROUND OF THE INVENTION

This invention relates to a synthetic resinous powder product to belaser-sintered in a selective laser sintering machine, such as aSINTERSTATION 2000 system manufactured and sold by DTM Corporation. Thelaser-sinterable powder (referred to as "sinterable powder" herein) is"designed" or "tailored" to incorporate specific physical propertiesuniquely adapted to form a bed (of powder) upon which a sintering laserin the infra-red region is directed.

Prior art sinterable powders are unable to yield a sintered part which,for most purposes, appears to be a duplicate of one which isisotropically molded. Moreover, conventional sinterable powders form abed which generally lacks the ability to provide the exigent heattransfer characteristics which determine whether a sintered part will bedistorted, even if it is successfully completed. Since a layer ofparticles typically rolled out of the feed bed and onto the part bed ofa selective laser sintering machine, is about 8 mils (200 μm) suchpowders used had a maximum particle diameter which was less than 200 μmand whatever "fines" were generated in the course of grinding the powderto the desired mesh size, irrespective of the distribution of particlesizes in the powder.

It has been observed that the selective laser sintering of amorphouspolymer powders typically results in finished parts that are somewhatporous. Typical amorphous polymers exhibit a second order thermaltransition at a temperature that is commonly referred to as the "glasstransition" temperature, and also exhibit a gradual decrease inviscosity when heated above this temperature. In the selective lasersintering of amorphous polymers, the part bed is maintained at atemperature near the glass transition temperature, with the powder beingheated by the laser at the part locations to a temperature beyond theglass transition temperature to produce useful parts, since viscositycontrols the kinetics of densification. While it may be at leasttheoretically possible to build fully dense (i.e., non-porous) partsfrom amorphous polymers, practical considerations arising from the useof high power lasers, such as thermal control, material degradation, andgrowth (undesired sintering of powder outside of the scanned regions)have prevented the production of such fully dense parts. Further, it hasbeen observed that the selective laser sintering of amorphous polymerpowders is also vulnerable to "in-build curl", where subsequent sinteredlayers added to the part shrink onto the solid substrate, causing thepart to warp out of the part bed.

The sinterable powders of the present invention are directed to yieldinga sintered article ("part") which, though porous, not only has theprecise dimensions of the part desired, but also is so nearly fullydense (hence referred to as "near-fully dense") as to mimic the flexuralmodulus and maximum stress at yield (psi), of the article, had it beenfully dense, for example, if it had been isotropically molded.

In addition, the properties deliberately inculcated in the sinterablepowder are unexpectedly effective to provide the bed with sufficientporosity to permit cooling gas to be flowed downwardly through it, yetmaintaining a quiescent bed in which the sintered part mimics theproperties of a molded article.

The term "near-fully dense" refers to a slightly porous article whichhas a density in the range from 80%-95% (void fraction from 0.2 to aslow as 0.05), typically from 85%-90% of the density (void fraction0.15-0.1) of a compression molded article which is deemed to be fullydense.

The term "fully dense" refers to an article having essentially nomeasurable porosity, as is the case when an article of a syntheticresinous powder is compression (or injection) molded from a homogeneousmass of fluent polymer in which mass individual particles have losttheir identity.

By a "quiescent bed" we refer to one upon the surface of which theparticles are not active, that is, do not move sufficiently to affectthe sintering of each layer spread upon a preceding slice sintered inthe part bed. The bed is not disrupted by the downward flow of gas, sothat the bed appears to be static.

To date, despite great efforts having been focussed on a hunt for theformulation of a sinterable powder which will yield a near-fully densepart, that formulation has successfully eluded the hunt. The goal istherefore to produce a mass of primary particles of a synthetic resinwhich has properties specifically tailored to be delivered by a rollerto the "part bed" of a selective laser sintering machine, then sinteredinto a near-fully dense prototype of a fully dense article.

A powder dispenser system deposits a "layer" of powder from a "powderfeed bed" or "feed bed" into a "part bed" which is the target area. Theterm "layer" is used herein to refer to a predetermined depth (orthickness) of powder deposited in the part bed before it is sintered.

The term "prototype" refers to an article which has essentially the samedimensions of a compression or injection molded article of the samematerial. The porous prototype is visually essentially indistinguishablefrom the molded article, and functions in essentially the same manner asthe molded article which is non-porous or fully dense. The flexuralmodulus, flexural strength and flexural elongation at yield, areessentially indistinguishable from the values obtained for a moldedarticle. One is distinguishable from the other only because theprototype has a substantially lower, typically less than one-half, theultimate tensile elongation (%), and notched Izod impact (ft-lb/in),than a compression molded article, though the prototype's tensilemodulus, tensile strength, and elongation at yield are substantially thesame as those of the compression molded article (see Table 1hereinbelow). In Table 1, the values given in square brackets are thestandard deviations under the particular conditions under which themeasurements were made.

The tensile elongation, ultimate (%), and notched Izod impact are lowerfor the prototype because of its slight porosity. Therefore the energyto break, which is the area under the stress curve up to the point ofbreak at ultimate elongation, is also very much lower than that for thecompression molded article. As is well known, any small imperfections ina homogeneous article will be reflected in the ultimate tensileelongation and notched Izod impact. However, confirmation that themolded article has been closely replicated is obtained by a comparisonof the fracture surfaces of the prototype and of the molded article.Photomicrographs show that these fracture surfaces of the prototype arevisually essentially indistinguishable from fracture surfaces of anisotropically molded non-porous part except for the presence of aprofusion of cavities having an average diameter in the range from 1μm-30 μm randomly scattered throughout said part, indicating similarcreep and fatigue characteristics. As one would expect, the cavitiesprovide evidence of the porosity of the prototype. Therefore it is fairto state that, except for the lower ultimate elongation or Izod impactof the prototype, due to its slight porosity, the prototype fails in thesame manner as the molded article.

A laser control mechanism operates to direct and move the laser beam andto modulate it, so as to sinter only the powder disposed within definedboundaries (hence "selectively sintered"), to produce a desired "slice"of the part. The term "slice" is used herein to refer to a sinteredportion of a deposited layer of powder. The control mechanism operatesselectively to sinter sequential layers of powder, producing a completedpart comprising a plurality of slices sintered together. The definedboundaries of each slice corresponds to respective cross-sectionalregions of the part. Preferably, the control mechanism includes acomputer--e.g. a CAD/CAM system to determine the defined boundaries foreach slice. That is, given the overall dimensions and configuration ofthe part, the computer determines the defined boundaries for each sliceand operates the laser control mechanism in accordance with the definedboundaries for each slice. Alternatively, the computer can be initiallyprogrammed with the defined boundaries for each slice.

The part is produced by depositing a first portion of sinterable powderonto a target surface of the part bed, scanning the directed laser overthe target surface, and sintering a first layer of the first portion ofpowder on the target surface to form the first slice. The powder is thussintered by operating the directed laser beam within the boundariesdefining the first slice, with high enough energy (termed "fluence") tosinter the powder. The first slice corresponds to a firstcross-sectional region of the part.

A second portion of powder is deposited onto the surface of the part bedand that of the first sintered slice lying thereon, and the directedlaser beam scanned over the powder overlying the first sintered slice. Asecond layer of the second portion of powder is thus sintered byoperating the laser beam within the boundaries which then define thesecond slice. The second sintered slice is formed at high enough atemperature that it is sintered to the first slice, the two slicesbecoming a cohesive mass. Successive layers of powder are deposited ontothe previously sintered slices, each layer being sintered in turn toform a slice.

Repetition of the foregoing steps results in the formation of alaser-sintered article lying in a "part bed" of powder which continuallypresents the target surface. If the particles of powder at theboundaries of each layer are overheated sufficiently to be melted,unmelted particles immediately outside the boundaries adhere to themolten particles within, and the desired sharp definition of the surfaceof the sintered article is lost. Without sharp definition at theboundaries, the article cannot be used as a prototype.

Particles of powder adjacent the surfaces of the article to be formedshould resist being strongly adhered to those surfaces. When particlesare not so strongly adhered they are referred to as "fuzz" because fuzzis easily dislodged from the surface, manually, and the dislodgedparticles retain most of their individual identities. Particles sotightly adhered to the surface as to be removed satisfactorily only witha machining step, are referred to as "growth". Such growth makes asintered part unfit for the purpose at hand, namely to function as aprototype for a compression molded part.

A method for sintering a powder into a shaped article in a selectivelaser sintering machine is disclosed in U.S. Pat. No. 4,247,508 toHousholder; U.S. Pat. Nos 4,863,538 and 5,132,143 to Deckard; U.S. Pat.No. 4,938,816 to Beaman et al.; and, U.S. Pat. No. 4,944,817 to Bourellet al., the relevant disclosure of each of which is incorporated byreference thereto as if fully set forth herein. "Sintering" is definedas the heating of the powder to a temperature which causes viscous flowonly at contiguous boundaries of its particles, with at least someportion of substantially all particles remaining solid. Such sinteringcauses coalescence of particles into a sintered solid mass the bulkdensity of which is increased compared to the bulk density of the powderparticles before they were sintered; and, a part formed by "slice-wise"joining of plural vertically contiguous layers which are sintered intostacked "slices" is therefore said to be autogenously densified. A layerof powder is confined by vertically spaced apart horizontal planes, nomore than about 250 μm apart and each slice is typically in the rangefrom 50 μm to 180 μm thick.

A specific goal of this invention is to produce a sinterable powder of asingle, that is, unblended, synthetic resin the molecular weight rangeand molecular weight distribution of which may be controlled to producea powder which, when exposed to the laser beam, is heated so that theouter portions of each particle have a narrowly defined range ofviscosity which results in the fusion of successive slices.

It must be remembered that before the powder can be sintered in the partbed, it must be delivered from the feed bed to the part bed upon whichthe powder is distributed in a thin, even layer about 125 μ thick, bythe roller of the selective laser sintering machine. Each distributedlayer should be thin and evenly distributed because the temperaturegradient through the cross-section of the sintered slice must be small,typically <5° C., more preferably <2° C., and most preferably <1° C. Tomeet this demanding criterion, the powder must be freely flowable fromthe feed bed onto the part bed.

By "freely flowable" we refer to a mass of small particles, the majorportion of which, and preferable all of which have a sphericity of atleast 0.5, and preferably from 0.7 to 0.9 or higher, so that the masstends to flow steadily and consistently as individual particles. Thoughsuch flow is conventionally considered a characteristic of a powderwhich flows through an orifice slightly larger than the largestparticle, such flow (through an orifice) is of less importance than theability of the powder to be picked up in the nip of a rotating rollerand transported by it as an elongated fluent mass of individualparticles urged along by the roller. A freely flowable powder has theproperty of being able to be urged as a dynamic elongated mass, referredto as a "rolling bank" of powder, by the rotating roller, even at atemperature near T_(s) the "softening point" of the powder.

At T_(s), the powder is on the verge of not being flowinglytransportable as a rolling bank against a rotating roller. By "softeningpoint" we refer to T_(s), at which a powder's storage modulus (G'_(s))has decreased substantially from its value of G' at room temperature. Ator above T_(s) the storage modulus G'_(s) of a sintered slice of thepowder is low enough so as not to let it "curl". By "curl" we refer tothe slice becoming non-planar, one or more portions or comers of theslice rising more than about 50 μm above the surface of the last(uppermost) slice in the horizontal x-y-plane.

A slice will curl when there is a too-large mismatch between thetemperature of the initial slice sintered by the laser and the bed ofpowder on which it lies; or, between powder freshly spread over ajust-sintered slice and the temperature at the upper interface of theslice and the freshly spread powder. Such a mismatch is the result of"differential heating". The importance of countering curl is mostcritical when the first slice is formed. If the first slice curls, theroller spreading the next layer of powder over the slice will push theslice off the surface of the part bed.

If the powder is transported from the feed bed to the part bed in whicha hot slice is embedded, and the temperature at the interface T_(i)between the hot upper surface of the slice and the freshly spread powderis high enough to raise the temperature of the freshly spread powderabove T_(s), this powder cannot be rollingly distributed over the hotslice because the powder sticks and smears over the hot slice. Theindication is that the slice is too hot.

If the powder in the feed bed is too cool, that is, so cool that theequilibrium temperature on the surface of the hot, embedded slice issuch that the temperature of the freshly spread powder is below T_(s),the slice will curl.

The slice will not curl when the powder spread over it reaches anequilibrium temperature at the interface, and the equilibriumtemperature is at or above T_(s). the precise temperature T_(i) at theinterface is difficult to measure, but to form successive slicescohesively sintered together, the temperature of the powder at theinterface must be above T_(s), but below the powder's "sticky point" or"caking temperature" T_(c) at which the powder itself will not flow.

By "sticky" we infer that the force required to separate contiguousparticles has exceeded an acceptable limit for the purpose at hand. Thiscaking temperature T_(c) may be considered to be reached when a criticalstorage modulus (G'_(s)) of the powder has been reached or exceeded. Thestorage modulus is a property of the powder akin to a material's tensilestrength and can be measured directly with a Rheometrics dynamicmechanical analyzer.

To form a sintered part in a selective laser sintering machine, aninitial slice is sintered from powder held in the part bed at near T_(s)but well below T_(c). By "near T_(s) " we refer to a temperature withinabout 5° C. of T_(s), that is T_(s) ±5, preferably T_(s) ±2.

Immediately after the initial slice is formed, the slice is much hotterthan the powder on which it rests. Therefore a relatively cool powder,as much as about 40° C., but more typically about 20° C. below itsT_(s), may be spread over the hot slice and the interface temperatureraises the temperature of the powder to near T_(s). As the powder isspread evenly over the hot slice it is to remain cool enough to bespread, but soon thereafter, due to heat transfer at the interface, mustreach or exceed T_(s), or the just-sintered slice will curl; that is,the temperature of the powder preferably enters the "window ofsinterability". This window may be measured by running two DSC(differential scanning calorimetry) curves on the same sample of powder,sequentially, with a minimum of delay between the two runs, one runheating the sample past its melting point, the other run, cooling thesample from above its melting point until it recrystallizes. Thedifference between the onset of melting in the heating curve, Tm, andthe onset of supercooling in the cooling curve, Tsc, is a measure of thewidth of the window of sinterability (see FIG. 6).

To ensure that the powder from the feed bed will form a rolling bankeven when it is rolled across the hot slice, the powder is usuallystored in the feed bed at a storage temperature in the range from 2° C.to 40° C. below the powder's T_(s) and transferred at this storagetemperature to the part bed, the feed bed temperature depending upon howquickly a layer of powder spread over a just-sintered slice enters thewindow of sinterability. The T_(s) may be visually easily obtained--whenthe powder is too hot to form a rolling bank, it has reached or exceededits T_(s).

It will now be realized that the cooler the powder (below T_(s)) thehigher the risk of curling, if the interface temperature is not highenough to raise the temperature of the layer of powder at least toT_(s). A commensurate risk accrues with a powder stored at too high atemperature. The storage temperature is too high, though the powderforms a rolling bank, when the powder smears or sticks as it traversesthe slice, an indication that the powder overlying the slice has notonly exceeded T_(s) but also reached (or gone beyond) T_(c).

Thus, though it is difficult to measure the interface temperature, or tomeasure T_(c) with a temperature probe, so as to measure the width ofthe window, it can be done visually. When the rolling bank of powdersticks or smears over the last-sintered slice, the T_(c) of the powderhas been reached or exceeded. Thus with visual evidence once candetermine the temperature range (T_(c) -T_(s)) which is the window ofsinterability or the "selective laser sintering operating window", soreferred to because the powder cannot be sintered successfully at atemperature outside this selective-laser-sintering-window. (see FIG. 6).

At the start of a sintering cycle it is best to maintain the temperatureof the upper layer of the part bed at T_(s), preferably 0.5°-2° C. aboveT_(s) so that the uppermost layer is presented to the laser beam in theselective-laser-sintering-window. After the first slice is formed, feedis rolled out from the feed bed at as high a temperature as will permita rolling bank of powder to be transferred to the part bed. The mostdesirable powders are freely flowable in a rolling bank at a temperatureonly about 5° C. below their T_(s).

However, as the mass of the sintered slices accumulates in the part bed,the sintered mass provides a large heat sink which transfers heat toeach layer of powder freshly spread over the hot mass, thus allowing arelatively cool powder, as much as 30° C., more typically 20° C., lowerthan T_(s) to be transferred from the feed bed, yet quickly come toequilibrium in the selective-laser-sintering-window as the layer isspread over the last preceding slice. Thus, when each layer is sintered,the later-formed slices will not curl.

It is important that the powder be "freely flowable" from the feed bed,preferably at a temperature sufficiently near T_(s) to ensure that thelast-sintered slice will not curl when the powder is spread upon it. Asalready pointed out above, if the first slice formed curls, no furtherprogress can be made. A fresh start must be made to sinter the part.

A powder is not freely flowable when the temperature at which it is heldor distributed exceeds its softening point. The powder cakes and doesnot flow at all when the caking temperature is reached. For example, onemay consider that at T_(c), G'_(s) decreases to a critical G'_(c), inwhich case the caking temperature T_(c) may also be referred to as the"G'_(c) temperature".

It is possible to transfer powder from the feed bed to the part bed atabove T_(s) if the impaired flowability allows one to do so, and therisk of operating too close to T_(c) is acceptable. Generally a powderdoes not form a rolling bank at or above its T_(s).

According to one aspect of the invention, it is preferred that thepowder used in the selective laser sintering process be sinterable in awide selective-laser-sintering-window. Though within narrow limits, the`width` (in °C.) of the window, varies from the start of the cycle andat the end (particularly when a large part is formed, as explainedabove). The width of the window also varies depending upon thecomposition of the powder. This width ranges from about 2° C. to about25° C.; more typically, it is about 5° C.-15° C. With a powder which isfreely flowable over a wide temperature range, one is able to form, inthe best mode, a solid, near-fully dense article when the powder issintered in a selective laser sintering machine which uses a roller tospread the powder.

The temperature at which G'_(s) is measured is believed to not becritical, provided the G'_(c) temperature offers an adequately largeselective-laser-sintering-window. Most desirable laser-sinterablepowders have an unexpectedly common characteristic, namely that thevalue of their G'_(c) is narrowly defined in the range from 1×10⁶dynes/cm² to 3×10⁶ dynes/cm₂.

For a crystalline powder (100% crystallinity), the softening point isits melting point Tm. Therefore G'_(s) and G'_(c) are essentiallyidentical and there is no G'-window. For an amorphous powder, itssoftening point is its initial glass transition temperature Tg. Anamorphous powder can offer a large window of sinterability but becauseits viscosity decreases too slowly as temperature increases and theG'_(c) limit of the selective-laser-sintering-window is approached, theviscosity is still too high. That is, the viscosity is too high to allowrequisite interchain diffusion at the boundaries of the particleswithout melting the entire particle. Therefore an amorphous powder isdifficult to sinter to near-full density, so that powders which qualifyas the product of this invention are semi-crystalline powders such asnylon, polybutylene terephthalate (PBT) and polyacetal (PA) whichprovide signs of crystalline order under X-ray examination, and show acrystalline melting point Tm as well as a glass transition temperatureTg. Because the crystallinity is largely controlled by the number anddistribution of branches along the chain, the crystallinity varies,bulky side chains or very long chains each resulting in a reduction ofthe rate of crystallization. Preferred polymers have a crystallinity inthe range from 10%-90%, more preferably from 15%-60%.

To summarize, the selective laser sintering process is used to make 3-Dobjects, layer-upon-layer sequentially and in an additive manner. Theprocess is more fully described in the '538 Deckard patent and comprisesthe following steps:

(1) Powder from the feed bed is "rolled out" by a roller, to a part bedwhere the powder is deposited and leveled into a thin layer, typicallyabout 125 μm (0.005") in depth.

(2) Following a pattern obtained from a two dimensional (2-D) section ofa 3-D CAD model, a CO2 laser "sinters" the thin layer in the targetregion of the part bed and generates a first slice of sintered powder ina two-dimensional ("2-D") shape. Directions for the pattern, and eachsubsequent pattern for successive slices corresponding to a desiredthree-dimensional ("3-D") prototype are stored in a computer-controller.It is critical for a slice-upon-slice construction of the prototype thatthe laminar, planar shape of each slice of sintered powder bemaintained, that is, "without curling".

(3) A second layer of powder from the feed bed is then deposited andleveled over the just-sintered layer in the part bed, forming a secondslice sintered to the first slice.

(4) The computer-controller makes incremental progress to the next 2-Dsection, the geometry of which is provided from the 3-D model, andinstructs the laser/scanner system to sinter the regions of the beddesired for successive 2-D sections.

(5) Still another layer of powder is deposited from the feed bed andleveled over the just-sintered layer in the part bed.

(6) The foregoing steps are repeated, seriatim, until all layers havebeen deposited and sequentially sintered into slices corresponding tosuccessive sections of the 3-D model.

(7) The sintered 3-D object is thus embedded in the part bed, supportedby unsintered powder, and the sintered part can be removed once the bedhas cooled.

(8) Any powder that adheres to the 3-D prototype's surface as "fuzz" isthen mechanically removed.

(9) The surfaces of the 3-D prototype may be finished to provide anappropriate surface for a predetermined use.

This invention relates mainly to producing and using a powder which isdesigned to satisfy the requirements of the first three steps of theprocess.

Although we have experimentally processed many synthetic resinouspowders in the selective laser sintering machine, we have found that fewmake near-fully dense parts. In most cases the measured values offlexural modulus and maximum stress at yield are at least 30% lower thanvalues obtained made by injection or compression molding the same part.We now understand, and have set forth below, what properties arerequired of a powder which can be successfully sintered in a selectivelaser sintering machine, and have accepted, at least for the time being,the many disappointing results we obtained with amorphous polymers suchas polycarbonate (PC) and acrylonitrile-butadiene-styrene resins (ABS).

It has now become evident that a semi-crystalline or substantiallycrystalline organic polymer is the powder of choice if it is to providethe high definition of surface ("lack of growth") which a prototype madefrom the tailored powder of this invention, provides.

By a "semi-crystalline polymer" or "substantially crystalline polymer"is meant a resin which has at least 10% crystallinity as measured byDSC, preferably from about 15-90%, and most preferably from about 15-60%crystallinity.

U.S. Pat. No. 5,185,108, issued Feb. 9, 1993, incorporated herein bythis reference, teaches that to produce a sintered article of wax havinga void fraction (porosity) of 0.1, a two-tier weight distribution of waxparticles was necessary. The desired two-tier distribution was producedby a process which directly generated a mass of wax microspheres suchthat more than half (>50%) the cumulative weight percent is attributableto particles having a diameter greater than a predetermined diameter(100 μm is most preferred for the task now at hand) for the particularpurpose of packing at least some, and preferably a major portion of theinterstitial spaces between larger particles, with smaller ones.

The two-tier distribution described in U.S. Pat. No. 5,185,108 wasarrived at by recognizing that the densest packing of uniform spheresproduces a void fraction (porosity) of 0.26 and a packing fraction of0.74 as illustrated in FIG. 1; and further, by recognizing that thepacking factor may be increased by introducing smaller particles intothe pore spaces among the larger spheres. As will be evident, thelogical conclusion is that the smaller the particles in the pore spaces,the denser will the packed powder (as illustrated in FIG. 2), and thedenser will be the part sintered from the powder.

As will further be evident, the greater the number of small particlesrelative to the large, in any two-tier distribution, the denser will bethe part. Since the goal is to provide a near-fully dense part, logicdictates that one use all small particles, and that they be as small ascan be.

However, a mass of such uniformly small particles is not freelyflowable. To make it freely flowable one must incorporate largerparticles into the mass, much in the same manner as grains of rice arecommonly interspersed in finely ground table salt in a salt shaker.Therefore, the tailored powder is a mixture of relatively very large andrelatively very small particles in a desirable two-tier particle sizedistribution for the most desirable sinterable powders.

The demarcation of size in the two-tier distribution and the ratio ofthe number of small particles to the number of large particles, setforth hereinbelow, are both dictated by the requirements of theselective laser sintering machine.

Further it was found that the rate of heat transfer into the mass of asmall particle is so much higher than that into the mass of a largeparticle, that one could not know either just how large the particles inthe upper tier should be, nor how many of such large particles could bepresent. If the heat transfer to small particles in the bed adjacent theboundaries of each layer was too high, unacceptable growth is generated.If the heat transfer is not high enough, the large particles, namelythose >53 μm, in the layer are not sintered, thus forming a defectiveslice. It is because essentially all these large particles are sinteredwithout being melted, and a substantial number of the small particles<53 μm are melted sufficiently to flow into and fill the intersticesbetween sintered large particles, that the finished sintered part isnear-fully dense. Under successful sintering conditions to form anear-fully dense part, the temperature of the powder must exceed T_(s)in less time than is required to melt the large particles >53 μm. If thetime is too long, large particles will melt and there will be growth onthe surfaces of the part; if the time is too short, all the largeparticles are not sintered. Thus the large particles not only help forma rolling bank, but also fill an important role to maintain the desiredtransient heat transfer conditions.

It has been found that only a substantially crystalline powder whichdoes not melt sharply, lends itself to the purpose at hand, and onlywhen the powder is stripped of substantially all too-large particles(termed "rocks") larger than 180 μm (80 mesh, U.S. Standard SieveSeries). By "substantially all" we mean that at least 95% of the numberof "rocks" in the powder are removed.

It has further been found that a laser-sinterable powder in the propersize range of from about 1 μm-180 μm, may, according to one aspect ofthe invention, be specified by (i) narrowly defined particle size rangeand size in a two-tier distribution, and, (ii) the"selective-laser-sintering-window".

According to another aspect of the invention to be described in detailhereinbelow, it has now been realized that the two-tiered particle sizedistribution is not absolutely necessary in order to create adistortion-free fully dense part in the selective laser sinteringprocess, provided that the recrystallization rate of the material issufficiently low.

Referring to the first aspect of the invention noted above, theunexpected effect of using the tailored powder with a definedselective-laser-sintering-window is supported by evidence of thesinterability of the powder in this window. Theselective-laser-sintering-window is directly correlatable to thepowder's fundamental properties defined by its G'_(c) temperature.

More surprising is that, despite the much larger number of smallparticles than large in the part bed, it is possible to flow the streamof cooling gas (nitrogen) downwardly through the quiescent bed at lowenough a pressure so as not to disturb the particles on and near thesurface of the bed sufficiently to cause movement noticeable by thenaked eye (hence referred to as "quiescent"). One would expect thepressure drop through a bed of very fine particles, more than 80% ofwhich are smaller than 53 μm (270 mesh) to be relatively high. But thepresence of the large particles, coupled with the fact that the powderis delivered from the feed bed and distributed evenly by a roller,rather than being pressed onto the bed, unpredictably provides therequisite porosity in the range from 0.4 to 0.55 to allow through-flowof a gas at superatmospheric pressure in the range from 103 kPa (0.5psig) to 120 kPa (3 psig), preferably from 107-115 kPa (1-2 psig) with apressure drop in the range from 3-12 kPa, typically 5-7 kPa, withoutdisturbing a quiescent part bed 30 cm deep.

The part bed formed by the tailored powder is unique not only becauseits specific use is to generate laser-sintered parts, but because thebed's narrowly defined porosity and defined particle size provides"coolability". In operation, the powder in the part bed is heated by amultiplicity of hot sintered slices to so high a temperature that thepowder would reach its caking temperature T_(c) if the hot bed could notbe cooled.

An identifying characteristic of a preheated `part bed` of powder havinga two-tiered distribution, with primary particles in the proper sizerange, stripped of rocks >180 μm, is that the bed is not too tightlypacked to permit the flow of cooling gas through the bed. Thischaracteristic allows the part bed to be maintained, during operationsintering a part, with a specified temperature profile which allowsformation of a distortion-free sintered part as it is formed slice-wise;and also, after the sintered part is formed, and the part lies in theheated part bed. By "distortion-free" is meant that no linear dimensionof the part is out of spec more than ±250 μm, and no surface is out ofplane by more than ±250 μm (20 mils).

Though the importance of a two-tier particle size weight distributionwas disclosed with respect specifically to wax particles in U.S. Pat.No. 5,185,108, it was not then realized that the ranges of particlesizes in each tier of the two-tier distribution controlled both, thedensity of the sintered part and the sinterability of the powder.Neither was it known that the distribution of particle sizes in atwo-tier distribution was as critical as the viscosity characteristicsof the material as a function of temperature.

The ranges of sizes in the two-tier distribution of particles used inthe powder according to this aspect of the invention is different fromthe ranges of the two-tier distribution of the wax powder described inU.S. Pat. No. 5,185,108. Quite unexpectedly, the formation of anear-fully dense sintered part requires that at least 80% of the numberof all particles in the bed are from 1 μm-53μ and that there besubstantially no (that is, <5%) particles greater than 180 μm (80 mesh)in a part bed. The importance of the few "large particles" to maintain(i) free-flowability near T_(s) and (ii) a predetermined temperatureprofile in a part bed while a sintered part is being formed,irrespective of the density of the part formed, to negate undesirable"growth" on the part, was not then known.

Because the "selective-laser-sintering-window" may be defined by therequirements of the selective laser sintering process, the part bed (andsometimes the feed bed) is heated to near T_(s) to negate the proclivityof the sintered layer to "curl". To minimize the curling of a slice asit lies on a part bed, it has been discovered that a preferredtemperature profile is to be maintained in the bed, with a slight butnarrowly specified temperature gradient on either side of a horizontalzone through the portion of the bed occupied by the sintered part,referred to as the "hot" zone.

The typical gradient in a part bed in a selective laser sinteringmachine is first positive, that is, the temperature increases to amaximum, then the gradient is negative, that is the temperaturedecreases from the maximum. The upper temperature gradient in the upperportion of the bed is positive, that is the temperature increases untilit reaches a maximum temperature T_(max) in the hot zone. The lowertemperature gradient in the lower portion of the bed is negative, thatis the temperature decreases from T_(max) in the hot zone to the bottomof the bed.

More specifically, the temperature in the upper portion of the bedprogressively increases as one moves downward from the upper surface ofthe bed to T_(max) ; then progressively decreases as one moves downwardfrom T_(max) to the bottom surface of the part bed, which surface is incontact with the bed-supporting piston.

The gradient in a conventional selective laser sintering machine withoutcontrolled gas-cooling of the part bed, in each direction is typicallygreater than 2° C./cm (5° C./in). Such a gradient was found to be toohigh to provide an acceptable risk of distortion of the part.

These considerations lead to temperature limits in the feed and partbeds which limits define the G'-window andselective-laser-sintering-window, namely, (i) the temperature at whichthe part bed is maintained, and the temperature profile therein, and(ii) the temperature at which the feed bed is maintained.

In turn, the temperature at which the part bed is maintained is definedby (a) a lower (minimum) part bed temperature below which curling is sopronounced as to negate any reasonable probability of effecting aslice-wise fusion of plural vertically contiguous slices; and, (b) anupper (maximum) temperature at which interparticle viscosity in the partbed makes it so "sticky" as to fuzz (obfuscate) the predeterminedboundaries of the part to be made. All sintered powder betweenvertically spaced apart lateral planes in the part bed is solidifiedsufficiently to have mechanical strength. The remaining unsinteredpowder remains freely-flowable.

The "improved" sinterable tailored powder provides not only thespecified particle size and two-tier distribution, but also a usable anddesirable selective-laser-sintering-window. The ability of a powdersimultaneously to satisfy each of the requirements, provides a measureof how "good" the chance that a powder will be sinterable in theselective laser sintering process to yield a near-fully dense, butporous article.

A major practical consequence of the narrowly defined window requiresthat the part bed be maintained at a specified temperature and with aspecified temperature profile so that each layer to be sintered lieswithin the confines of the selective-laser-sintering-window. A differenttemperature, whether higher or lower, and/or a different temperatureprofile, results in regions of the just-sintered initial slice of powderwhich will either cause a sintered slice to melt and be distorted in alayer of the part bed which has "caked"; or, will cause a sintered sliceto curl if the part bed temperature is too low. In the past this hasbeen an all too common occurrence with the result that an undesirablepart was made. The tailored powder and unique bed which it forms nowmake production of an unacceptable part an uncommon occurrence.

SUMMARY OF THE INVENTION

A laser-sinterable semi-crystalline synthetic resinous powder (referredto as a "tailored powder"), having defined parameters of particle sizedistribution, molecular weight range, molecular weight distribution andcrystallization characteristics is found to overcome the disadvantagesof known powders used to form a sintered part in a selective lasersintering machine. The unexpected effect of providing a sinterablepowder which has a defined selective-laser-sintering-window is evidencedin the ability to predict the sinterability of the powder with a lasergenerated at a wave-length which is absorbed sufficiently to heatparticles of the powder to their critical storage modulus G'_(c) whenthe outer portion of the particles have the viscosity required to becohesively sintered.

According to a first aspect of the invention, the two-tier particle sizedistribution and the number average ratio of particles smaller than 53μm be >80%, that is, more than 80% of all the particles in the powder besmaller than 53 μm, allow the powder to be freely flowable onto the partbed so as to be presented to the laser beam in theselective-laser-sintering-window, and also to form a bed of desiredporosity which (i) allows passage of a low pressure inert cooling gas tokeep the bed from overheating, and (ii) provides the desired absorptionof infra-red energy from the laser beam to yield a near-fully denseslice. A specified temperature profile is maintained in the part bedwith the flow-through inert cooling gas stream, but the tailoredselective laser sintering powder is sintered with a conventionalselective laser sintering protocol. The powder yields a sintered articlewhich is porous but so near-fully dense that the porous article hasstrength characteristics which unexpectedly mimic (are substantially thesame as) those of an isotropically molded, fully dense article of thesame powder.

It is therefore a primary object of this invention to provide anear-fully dense part in a selective laser sintering machine, the partformed from a semicrystalline or substantially crystalline syntheticresinous sinterable powder having tailored properties uniquely adaptedto the purpose at hand.

According to one aspect of this invention, it is a general object ofthis invention to provide a bed of tailored powder of a semi-crystallineunblended polymer having the following physical properties:

(a) a major portion by weight of the powder, and preferably essentiallyall the powder having a sphericity in the range from greater than 0.5 to0.9, and a two-tier particle size distribution of primary particleshaving an average diameter smaller from than 180 μm, with substantiallyno particles >180 μm, provided further that the number average ratio ofparticles smaller than 53 μm is greater than 80%, preferably greaterthan 90%, and most preferably greater than 95%, the remaining particlesbeing in the size range from 53 μm to 180 μm; a layer of the powder nomore than 250 μm deep absorbs essentially all infra-red energy at the10.6 μm wavelength beamed therethrough, and absorbs more than 50% ofthat energy in a layer no more than 180 μm thick;

(b) a crystallinity in the range from 10% to 90%, preferably from 15% to60%, a number average molecular weight in the range from about 30,000 to500,000, preferably 60,000-300,000, and a molecular weight distributionin the range from 1 to 5, preferably from 1.2 to 3; and,

(c) a "selective-laser-sintering-window" in the temperature range from2° C.-25° C. between the softening temperature T_(s) of the powder andits "caking temperature" T_(c), such that the powder has a "flow time"of <20 sec for 100 g in a funnel test (ASTM D1895-61T) at a temperaturenear T_(s) in a range from 70° C. to 220° C., but below the powder'sT_(c) ; and,

(d) a melt viscosity in the range from 100-10⁴ poise (10-1000 Pa-sec)when the temperature of the powder being sintered exceeds T_(c) in lesstime than is required to melt contiguous large particles >53 μm.

The numerical value of the storage modulus G'_(s) for the tailoredpowder is much lower than the value of G' at room temperature, and thetemperature at which G'_(s) is measured is in preferably the range from5° C. to 25° C. below the G'_(c) temperature of the powder.

It is also a general object of this invention to provide a bed oftailored powder in a laser-sintering zone, the bed having the foregoingdefined characteristics which are evidenced in:

(i) a "selective-laser-sintering-window" in the range from T_(s) toT_(c) ; and,

(ii) a `part bed` in which the sintered part is removably embedded whileit dissipates heat to generate a temperature profile defined bysequential positive and negative temperature gradients, in a verticalplane through the part bed; such a gradient occurs when the uppermostslice is less than 250 μm thick, and is near T_(s) of the powder, andthe temperature of the sintered part is near T_(c). Further, thegradient from the upper surface of the part bed to the maximumtemperature in the horizontal zone in which the sintered part lies, ispositive, the temperature increasing at a rate in the range from 0.2°C./cm (0.5° C./in) to 2° C./cm (5° C./in) of vertical depth; and, fromthe maximum temperature in the horizontal zone, to the bottom of thebed, the gradient is negative, the temperature decreasing at a rate inthe range from 0.2° C./cm (0.5° C./in) to 2° C./cm (5° C./in).

It has also been discovered that the tailored powder which isfree-flowing at an elevated temperature below its Tg or Tm, typically atfrom 30° C. below T_(c), but with some powders, as little as 2° C., isuniquely adapted to yield, when sintered by a laser beam, a near-fullydense, laser-sintered article having a density in the range from80%-95%, typically from 85%-90% of the density of a compression moldedarticle which is deemed to be fully dense, and the mode of failure, whenfractured in bending, is essentially identical to the mode of failure ofan isotropically molded article of the same powder, except for cavitiescorresponding to the porosity of the sintered article. The sinteredarticle may have some unsintered particles ("fuzz") adhering to itssurface, but the fuzz is removable by lightly abrading the surfacewithout changing the contours of the near-fully dense sintered article.

It is therefore another general object of this invention to produce alaser-sinterable polymer powder consisting essentially of an unblendedpolymer having substantially no particles >180 μm in a mass of particlesin which the number average ratio of particles in the range from 1 μm-53μm is greater than 80%, the remaining particles being in the size rangefrom 53 μm to 180 μm; and, substantial crystallinity in the range from25% to 95%, which provides a selective-laser-sintering-window of from 2°C. to 25° C., and which powder when sintered in a bed with a specifiedtemperature profile, allows each layer of powder, in the range fromabout 50 μm (2 mil) to about 250 μm (10 mils) thick, to be sinteredwithout curling.

It is a specific object of this invention to provide a laser-sinterableunblended polymer powder tailored to have the aforespecified two-tierdistribution of primary particles which have a sphericity in the rangefrom greater than 0.5 to 0.9, a bulk density of 500 to 700 g/L, andcrystallinity in the range from 15 to 90%; has a "flow time" as given,at near T_(s) but 2° C. to 25° C. below the powder's caking temperatureT_(c) ; and a specified melt viscosity (shear viscosity) >10 Pa-sec,typically in the range from 10 pa-sec to 1000 Pa-sec, when thetemperature of the powder being sintered exceeds T_(c) in less time thanis required to melt contiguous large particles >53 μm; provided furtherthat the pressure drop through a quiescent part bed 38 cm deep with agas flow of 3-10 L/min through the bed is less than 10 kPa. The amountof gas flowed is not narrowly critical provided it is insufficient tocause channeling in the bed, or otherwise disrupt the bed, andsufficient to maintain the desired temperature profile in the bed.

According to another aspect of the invention, it has been found that thetwo-tiered particle size distribution is not required for the creationof a near-fully dense part, with minimal dimensional distortion, formaterials and conditions where the recrystallization rate issufficiently low. In this regard, it has been discovered that the rateof crystallization of the semi-crystalline organic polymer is a keyproperty in controlling curl and achieving dimensional control in thesintered part. Materials that recrystallize relatively slowly aftermelting exhibit sufficient dimensional stability and create near-fullydense, distortion-free parts in the selective laser sintering process.Specifically, polymers that show little or no overlap between themelting and recrystallization peaks when scanned in a DSC at typicalrates of 10°-20 C./minute work best in the selective laser sinteringprocess.

It is therefore another object of this invention to provide alaser-sinterable polymer powder that resolidifies sufficiently slowly toeliminate in-build curl and in-plane distortion in parts produced by theselective laser sintering process.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied with schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

FIG. 1 is a schematic illustration of a bed of uniform spheres packed ina bed.

FIG. 2 is a schematic illustration of a bed of large spheres and verysmall ("too-small") spheres, showing that the too-small particles fitwithin the interstitial spaces between larger particles, and produce abed of higher hulk density and correspondingly higher pressure drop.

FIG. 3 is a graphical presentation of the number distribution of aparticular tailored powder, namely Nylon 11.

FIG. 4 a graphical presentation of the volume distribution of the samepowder for which the number distribution is illustrated in FIG. 3.

FIG. 5 is a schematic illustration of an elevational cross-sectionalview of a cylindrical part bed of a selective laser sintering machineshowing the position of the bed-supporting cylinder near the top of thecylinder at the beginning of the sintering procedure, and after thesintered part is formed; along with indications of the temperatureprofile within the bed for the tailored powder of this invention usedwith a conventional selective laser sintering procedure (on the left)without exteriorly controlling the temperature profile; and for thetailored powder with exterior temperature control of the bad temperatureprofile (right hand side).

FIG. 6 shows DSC scans for the heating and cooling curves of alaser-sinterable PBT powder.

FIGS. 7A and 7B show heating and cooling DSC scans for wax, taken at 20°C./minute, showing the overlap between the melting and recrystallizationpeaks.

FIGS. 8A and 8B show heating and cooling DSC scans for Nylon 11, takenat 10° C./minute, showing the lack of overlap between the melting andrecrystallization peaks.

FIGS. 9A 9B eating and cooling DSC scans for SC 912 powder, taken at 10°C./minute, showing little overlap between the melting andrecrystallization peaks.

FIG. 10 shows heating and cooling DSC scans for SC 912 powder, taken at10° C./minute, showing an alternate method of characterizing the littleoverlap between the melting and recrystallization peaks.

FIGS. 11A and 11B show heating and cooling DSC scans for AffinitySM-1300 powder, taken at 10° C./minute, showing little overlap betweenthe melting and recrystallization peaks.

FIG. 12 shows heating and cooling DSC scans for Affinity SM-1300 powder,taken at 10° C./minute, showing an alternate method of characterizingthe little overlap between the melting and recrystallization peaks.

FIG. 13 shows heating and cooling DSC scans for IP60 powder, taken at10° C./minute, showing the overlap between the melting andrecrystallization peaks.

FIG. 14 shows heating and cooling DSC scans for Surlyn 8660 powder,taken at 10° C./minute, showing the overlap between the melting andrecrystallization peaks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The selective laser sintering machine specifically referred to hereinuses a 10.6 μm CO₂ laser, but any other infra-red laser-generatingsource may be used, as well as excimer lasers and neodymium glass laserswhich generate in the near-infra-red. A preferred laser is a SynradModel C48-115 CO₂ laser. Powders are sintered using a 10.6 μm laser inthe range from 3 to 30 watts operated at a ratio of power/scan speed inthe range from 0.075 watts/cm/sec to 0.3 watts/cm/sec, preferably in therange from 0.1-0.2 watts/cm/sec using a beam width in the range from0.23-0.9 mm, preferably from 0.4-0.6 mm. Particularly with Nylons 6, 11and 12, PBT and PA, the selective laser sintering machine is preferablyoperated at a fluence in the range from 1 mJ/mm² to 100 mJ/mm², morepreferably from 15 mJ/mm² to 45 mJ/mm², where fluence (milliJoules/mm²)is the measure of the energy of the laser beam delivered to a definedarea of powder. The laser is typically operated with a beam width of 0.6mm and a power ranging from 3-22 watts, preferably from 5-10 watts, at ascanning speed ranging from about 76.2 cm/sec to 178 cm/sec.

Among the requirements of a preferred semi-crystalline or substantiallycrystalline powder uniquely tailored to yield, when sintered, a porousbut near-fully dense article, are the following:

Free-Flowing or Non-Caking

The powder is freely flowable and does not cake when stored for up to 24hr, at a temperature in the range from 2° C. to 20° C. below its cakingtemperature; in particular, the powder does not cake even when held from1-8 hr in the part bed at T_(s), at a slightly higher temperature thanits storage temperature in the feed bed which latter is lower than T_(s)of the powder. A determination as to whether a powder will meet thefree-flowability requirement is made by running the time-temperatureASTM flow test referred to hereinabove.

Maximum Particle Size and Sphericity

The powder has sphericity >0.5 and contains essentially no particleshaving a nominal diameter exceeding 180 μm.

Referring to FIG. 3 there is presented in graphical form the numberaverage particle distribution of Nylon 11. This powder was produced fromNylon 11 (lot #R256-A02) having a number average molecular weight in therange from 75,000 to 80,000 and a molecular weight distribution in therange from 1.2-1.7.

The Nylon 11 was ground in a manner which produces a mixture ofrelatively coarse particles having a sphericity greater than 0.5 and awide distribution of smaller particles. The mixture was then sieved toeliminate substantially all particles larger than 180 μm, and theremaining particles were classified so as to conform to the numberdistribution shown in FIG. 3. The size distribution of the particles isobtained with a Malvern instrument which measures the size of theparticles with a laser beam.

Flowability in the Selective-Laser-Sintering-Window

The melt viscous flow of polymer powder on the surface of a slice heatedby the laser is determined by maintaining a temperature balance. Forgood interdiffusion of the polymer chains (sufficient to provideparticle-to-particle adhesion, and layer-to-layer adhesion), a low meltviscosity is desirable. However, part definition is lost if significantmelting occurs because the melt cannot be tightly confined nearboundaries of the part being formed. Sintering is effected without meansto assure isolation of the desired part features.

The selective-laser-sintering-window is of importance at this step (andstep 1) because the temperatures of both beds, the feed bed and the partbed are elevated. Since the temperature of the part bed is elevated tothe softening point of the powder to minimize curling, the wider theselective-laser-sintering-window, the greater the processing latitudeprovided by the powder. Maintaining the balance of properties in atailored powder permits the requisite particle-to-particle fusion withina layer, and also layer-to-layer fusion, both of which are necessary tomake a porous but near-fully dense part.

Referring to FIG. 4 there is shown a volume distribution curve of thesame particles for which the number distribution is illustrated in FIG.3, to show why the powder is freely flowable and how much of the volumein a bed of particles is occupied by "large particles". It appears thatthe few large particles are mainly responsible for rolling out the smallparticles with them, and also for permitting the essentiallyunobstructed passage of inert gas downwardly through the bed.

Growth

Since the finished (sintered) three-dimensional (3-D) part(s) are formedin the part bed in which the unsintered powder provides mechanicalsupport for the sintered part, the part is subject to the thermalchanges in the part bed due to the presence of the sintered part.Sequential, sudden heating of successive slices of powder in a thermallyinsulated environment causes the bed temperature to rise. The insulatingenvironment is due to the sintered part being surrounded by a mass ofporous powder which is a good insulator. When the temperature around thesintered part is either not low enough, or too high, the sintered partwill distort due to thermal stresses in the bed. In addition, if thesurfaces of the hot sintered part are too hot, there are agglomerationsof fused particles adhering to and scattered as "growth" over thesurfaces of the finished part, which growth must be removed and this canusually only be done by machining the growth away. When some "growth"does occur with the use of a tailored powder, the growth is so slightthat it can be removed without damage to the surfaces of the part sothat the surfaces are smooth to the touch. If there is substantialgrowth, the part made is scrapped.

The benefit of large particles in the two-tier distribution, accordingto this aspect of the invention, will be understood when it is realizedthat too-small particles, if not rollingly deposited on the part bed,would get packed and obstruct flow of the inert gas. The effect of beingrollingly deposited layer upon layer, referred to as "layer-wise", ontothe surface of the bed results in a "fluffy" bed which is dynamicallystable but quiescent and relatively porous. The bed densities of apowder when not rollingly deposited are typically at least 20% higherthan that of a bed of rollingly deposited powder.

A bed of such particles, when packed, are more quickly heated andover-heated (because of their small mass). The over-heated particles arethen easily fused to the surface of the sintered part as "growth". Theimportance of controlling the top-to-bottom temperature profile withinthe part bed will be better understood by reference to FIG. 5.

The preferred crystallinity of a tailored powder which produces anear-fully dense sintered part with minimal growth is that which iscorrelatable to an observed heat of melting by DSC in the range from20-120 cals/gm preferably from 30-60 cals/gm.

Referring to FIG. 5 there is shown schematically, in cross-sectionalview, a cylindrical part bed referred to generally by reference numeral10, having sidewalls 11 and a bottom 12 through the center of which isslidably inserted a piston rod 13 having a piston 14 with a flathorizontal surface which supports a bed of thermooxidatively degradablepowder 20. A through-passage having a relatively large diameter in therange from about 2.5 cm to 3.5 cm has a porous sintered metal disc 15press-fitted into it to provide essentially free-flow of an inert gas,preferably nitrogen or argon, through it. A typical part bed has adiameter of 30.5 cm, and the travel of the piston from the bottom 12 tothe top of the walls 11 is 38.1 cm.

A cylindrical part 30 with tapered ends, the bottom being truncated, isformed by sintering layer upon layer of preheated tailored powder,starting with the piston in the position indicated by its phantomoutline at 14', supporting a bed of preheated powder about 10 cm deep,indicated by the depth d₁. The powder and walls of the cylinder areheated by infrared heating means to keep the temperature of the bedabout 10° C. below the sticky temperature of the powder. However, it isdifficult to heat the piston within the cylinder so that the piston istypically at a slightly lower temperature than the powder. Further, themass of the piston provides a heat sink to which the bottom layer ofpowder dissipates heat faster than any other layer. The upper surface ofthe bed is in the same plane as the top of the cylinder over which theroller (not shown) of the selective laser sintering machine distributespowder from the feed bed (also not shown).

As layer upon layer of powder is sintered, forming sequential horizontalslices of the sintered part 30, the piston 14' moves downwards untilfinally the part is completely sintered. The sintered part 30 is thussupported on the bed of powder on the bottom, and the depth of thislower portion of the bed is indicated as being b₁. This bed is the sameinitially presented as the target, and its depth b₁ remains numericallyequal to the depth d₁ when the piston 14 has moved down to a depthindicated by d₂. The sintered part 30 rests on the bed of powder b₁thick, the bottom of the sintered part being at a depth d₃.

Referring now to the result of a conventional selective laser sinteringprocedure, there is formed a hot sintered part 30 dissipating heat tothe powder 20 surrounding it in unsteady state heat transfer. The lowerportion b₁ forms a relatively cool zone of powder which dissipates heatto the piston 14, and through which powder heat from the part 30 isrelatively well dissipated by convection currents through the bed b₁.

As soon as sintering is completed, the upper portion of the bed havingdepth d₄, particularly near the surface, begins to dissipate heat frompart 30 lying within upper portion d₄. Heat dissipated by the part 30 istransferred relatively well mainly by convection currents through theupper portion d₄ of the powder bed 20, and less effectively throughlower portion b₁.

The portion of the sintered part 30 lying in the intermediate portion ofthe bed 20, that is, the portion between the bed depths d₁ and b₁, isrelatively well insulated by the surrounding powder. Heat from the part30 causes the temperature to rise so that a maximum temperature T_(max)is reached. The temperature at the surface of the relatively quicklycooling upper portion of the bed, is indicated by T_(min1) and thetemperature of the relatively quickly cooling lower portion of the bedb₁ is indicated by T_(min2). It is thus seen that a temperature profileis established in the bed, the maximum temperature being substantiallyabove the lowest temperatures in the profile, and located in ahorizontal zone intermediate the upper and lower surfaces of the bed.

In the conventional selective laser sintering procedure, using the noveltailored powder, there is no forced cooling of the heated bed with gasso that a typical gradient between T_(min1) and T_(max), and betweenT_(max) and T_(min2) is more than 2° C./cm in each case (on either sideof T_(max)). For example, if T_(min1) at the upper surface aftersintering is 175° C., T_(max) is 182° C. and T_(min2) is about 171° C.Because T_(max) is very close to the melting point 183° C. of thepowder, the sintered part is exposed to a high likelihood of beingdistorted. It will be evident that a large part of this powder could notbe sintered successfully in a conventional selective laser sintering bedbecause T_(max) will exceed T_(c) and the part will distort.

In FIG. 5, on the left hand side thereof, the straight lines drawnconnecting the temperatures at the surface and bottom of the bed, aredrawn on the assumption that the gradient is a straight line, which itmost probably is not, but the linear representation serves as anapproximation to focus the fact that the gradient is steeper for theconventional selective laser sintering procedure than it is for thenovel procedure in which an inert cooling gas is flowed through the bedwhile the part is being sintered.

In the procedure with forced cooling, using the novel tailored powder,the porosity of the bed permits forced cooling of the heated bed withinert gas, so that a typical gradient between T_(min1) and T_(max), andbetween T_(max) and T_(min2) is less than 2° C./cm in each case. Forexample, if T_(min1) at the upper surface after sintering is 175° C.,T_(max) is 177° C. and T_(min2) is about 173° C. Because T_(max) is notclose to the melting point 183° C. of the powder, the sintered part isnot likely to be distorted.

The temperature profile for the process conditions using the inertcooling gas are shown on the right hand side of FIG. 5, where it is seenthat the gradient to T_(max) is less, and T_(max) itself is lower thanit was in the conventional selective laser sintering process. Thus, therisk of part distortion and growth (on the surface) is minimized as isthe thermal degradation to the powder surrounding the sintered part.Such thermal degradation occurs when the powder is overheated, that is,too far past its softening point, even if it is not heated past itsG'_(c) temperature.

To put the foregoing details in perspective, one may evoke a physicalpicture of the selective-laser-sintering-window by reference to FIG. 6in which curve A (plotted with squares to track heat flow) representsthe cooling curve for a sample of tailored PBT powder. The peak occursat 193° C., but supercooling commences near the temperature 202° C., apoint indicated by the arrow C (T_(s)). Curve B (plotted with circles)represents the heating curve for the same sample. The peak occurs at224° C., but onset of melting commences near the temperature 212° C., apoint indicated by the arrow M (T_(c)). Thus, the window W is providedby the difference in the temperatures at M and C, which for this sampleof PBT is 10° C.

The following results were obtained when Nylon 11 having a molecularweight Mn of about 80,000; Mw/Mn=1.6, and G'_(c) =2×10⁶ dynes/cm² at175° C. was sintered into test bars with a beam width of 0.6 mm, thelaser power set at 8 watts and a scan speed of 175 era/sec. The valuesfor four sets of bars were averaged in Table 1 hereinbelow.

Other preferred semi-crystalline polymers which are successfullytailored for use in the selective laser sintering machine arepolybutylene terephthalate (PBT); polypropylene (PP); and polyacetal(PA). The preferred mean primary particle diameter for each of thetailored powders is in the range from 80 μm-100 μm. The values for thesepowders are given in the following Table 2.

                  TABLE 2                                                         ______________________________________                                                                   selective-laser-sintering-                         Powder   T.sub.s, °C.                                                                     T.sub.c, °C.                                                                   window, °C.                                 ______________________________________                                        Nylon 11 153       170     17                                                 PBT      195       210     15                                                 PA       150       157     7                                                  ______________________________________                                    

Each of the foregoing tailored powders was used to make sintered bars 10cm long, 2.5 cm wide and 3 cm thick. A statistically significant numberof bars were made from each powder and tested to compare the sinteredbars with bars of identical dimensions but compression molded. Theresults with PBT are set forth in the following Table 3:

                  TABLE 3                                                         ______________________________________                                        Comparison of Physical Properties of                                          Sintered and Compression Molded Test Bars of PBT                                            Sintered Injection Molded*                                      ______________________________________                                        Density, g/cm.sup.3                                                                           1.19       1.31                                               Flexural Modulus, psi                                                                         2.99 × 10.sup.5                                                                    3.80 × 10.sup.5                              Max. Stress at yield, psi                                                                     8.3 × 10.sup.3                                                                     14.7 × 10.sup.3 **                           Notched Izod, ft-lb/in                                                                        0.29       0.70                                               HDT, °C. 206        163**                                              ______________________________________                                         *supplier's data  no compression molded data available.                       **value of max stress yield for injection molded sample would be higher       because of chain orientation; value of HDT is different because the sampl     preparation and thermal history is different from applicant's sample.    

The conditions for sintering test bars from several differentsemicrystalline materials, each of which having a different window ofsinterability is provided in the following Table 4 hereinbelow. In eachcase, the selective laser sintering machine was operated with a laserhaving a beam width of 0.6 cm, at its maximum power (22 watts) and ascan speed of from 127-178 cm/sec (50-70 in/sec), maximum power beingused so as to finish sintering test bars in the least possible time. Ineach case the bars were sintered in a part bed having a diameter of 30cm which can hold powder to a depth of 37.5 cm. In each case, the powderwas maintained in the feed bed at below T_(s) and the powder wastransferred by a roller to the part bed, the surface of which was nearT_(s). In each case, the bed was heated by an external electric heaterto bring it up to temperature. In each case, note that the density ofthe sintered part is about 90% of the density of a molded, fully dense,part. Even better physical properties are obtained when the parts aresintered at lower power and slower scan speed (lower fluence).

According to another aspect of the present invention, it has now beendiscovered that the rate of crystallization of the semi-crystallineorganic polymer is a key property in controlling curl and achieving"in-plane" (x-y) dimensional control. In the selective laser sinteringprocess, the part bed temperature can usually be maintained just belowthe onset of melting the semi-crystalline powder. At the melting point,the material is transformed from a solid to a viscous liquid over anarrow temperature range. Only a small quantity of energy (the heat offusion) is required to transform the material to a state wheredensification can occur. Not all semi-crystalline polymers work well inthe selective laser sintering process, however. Materials thatresolidify or recrystallize quickly after melting tend to exhibitin-build curl, just like amorphous materials. Wax is an example of amaterial that recrystallizes so quickly that it develops in-build curl.To build flat wax parts in the selective laser sintering process,support structures which anchor the parts to the piston bed arerequired.

Some materials, however, resolidify slowly enough at the part bedtemperature (i.e., the driving force for crystallization is small enoughnear the melting point) that the parts remain in the supercooled liquidstate for a significant amount of time during the part building process.Since liquids do not support stresses, no in-build curl is observed aslong as the part is not cooled sufficiently to induce more rapidrecrystallization. Nylon 11 is an example of a material thatrecrystallizes sufficiently slowly in the selective laser sinteringprocess to eliminate in-build curl. During the building of Nylon 11parts in the selective laser sintering process, the parts remaintransparent to depths of greater than one inch. This transparencyindicates that little or no resolidification or recrystallization of thepart has occurred, since resolidified, semi-crystalline parts areopaque.

The rate of crystallization can also be characterized by DSC. Whileactual rates of crystallization are often difficult to quantify fromthese experiments, the difference in temperature between the onset ofmelting and onset of recrystallization is directly related to the rateof crystallization--the larger this temperature difference, the slowerthe rate of crystallization. As discussed hereinabove with respect tothe "window of sinterability," to create a DSC trace, a material isheated to above its melting point at a controlled rate and then cooledback down, also at a controlled rate. This observed temperaturedifference between melting and recrystallizing, however, can also beaffected by the heating and cooling rates used to create the DSC data.Data must therefore be reported in terms of scanning rate. Specifically,polymers that show little or no overlap between the melting andrecrystallization peaks when scanned in a DSC at typical rates of10°-20° C./minute work best in the selective laser sintering process.

FIGS. 7A and 7B show heating and cooling curves, respectively, for wax,taken at a rate of 20° C./minute. FIG. 7A shows a heating curve for asample of wax powder where, as the crystalline phase melts, anendothermic peak is observed. FIG. 7B shows a cooling curve for the samesample of wax where, when cooled, an exothermic peak is observed as thematerial recrystallizes. Note that the melting and recrystallizationpeaks shown in FIGS. 7A and 7B overlap significantly-from about 40° C.to about 60° C. FIGS. 7A and 7B thus indicate that wax recrystallizesrelatively quickly when cooled to a temperature just below its meltingpoint. This rapid recrystallization causes in-build curl in theselective laser sintering process, unless special precautions are taken.

FIGS. 8A and 8B show heating and cooling curves, respectively, for Nylon11, taken at a rate of 10° C./minute. FIG. 8A shows a heating curve fora sample of Nylon 11 powder. FIG. 8B shows a cooling curve for the samesample of Nylon 11 powder. Note that the melting and recrystallizationpeaks shown in FIGS. 8A and 8B do not overlap at all. FIGS. 8A and 8Bindicate that Nylon 11 recrystallizes upon cooling at a temperaturesignificantly lower than its melting point. Thus, Nylon 11 remains inthe liquid state relatively longer than wax at temperatures below themelting point of the respective materials. Because liquids do notsupport stresses, Nylon 11 therefore does not exhibit in-build curl inthe selective laser sintering process.

FIGS. 9A and 9B show heating and cooling curves, respectively, for SC912, a polypropylene copolymer powder sold by Montel. FIG. 9A shows aheating curve for a sample of SC 912 powder as shown by DSC measured ata scanning rate of 10° C./minute. FIG. 9B shows a cooling curve for asample of SC 912 powder as shown by DSC measured at a scanning rate of10° C./minute. The melting and recrystallization peaks shown in FIG. 9Aand 9B exhibit an overlap of approximately 13° C., from about 120° C. toabout 133° C. Such little degree of overlap indicates that SC 912 powderrecrystallizes sufficiently slowly in a selective laser sinteringprocess to eliminate in-build curl.

Alternatively, this sufficiently slow recrystallization rate of SC 912powder can be expressed as a percentage ratio of the area under themelting peak below the temperature at which SC 912 begins torecrystallize during cooling to the total area under the melting peak.FIG. 10 shows a heating curve and a cooling curve for a sample of SC 912powder as shown by DSC measured at a scanning rate of 10° C./minute. Oneshould note that in FIG. 10, the endothermic melting peak pointsdownward and the exothermic cooling peak points upward, as the displayconvention of the DSC apparatus used to generate FIG. 10 was directlyopposite from the convention of the DSC apparatus used to generate FIGS.9A and 9B. As shown by the cooling curve of FIG. 10, the sample of SC912 begins to recrystallize at approximately 143° C. The total areaunder the melting peak is measured as approximately 74.0 Joules/gram.The area under the melting peak below this onset of recrystallizationtemperature, which is shaded and labeled with the letter A in FIG. 10,is measured as approximately 4.6 Joules/gram. Therefore, approximately6.2% of the total area under the melting peak is below the onset ofrecrystallization temperature. Such little degree of overlap between themelting peak and the recrystallization peak indicates that SC 912recrystallizes sufficiently slowly in a selective laser sinteringprocess to eliminate in-build curl.

FIGS. 11A and 11B show heating and cooling curves, respectively, forAffinity SM-1300 (hereinafter "SM-1300"), a single site, branchedpolyethylene copolymer powder sold by Dow Chemical. It is believed thatSM-1300 is a copolymer of ethylene and octene. FIG. 11A shows a heatingcurve for a sample of SM-1300 powder as shown by DSC measured at ascanning rate of 10° C./minute. The portion of the heating curve fromabout 45° C. to about 55° C. represents the glass transition temperaturefor the powder. FIG. 11B shows a cooling curve for a sample of SM-1300powder as shown by DSC measured at a scanning rate of 10° C./minute. Themelting and recrystallization peaks shown in FIGS. 11A and 11B exhibitan overlap of approximately 11° C., from about 75° C. to about 86° C.Such little degree of overlap indicates that SM-1300 recrystallizessufficiently slowly in a selective laser sintering process to eliminatein-build curl.

Alternatively, this sufficiently slow recrystallization rate of SM-1300powder can be expressed as a percentage ratio of the area under themelting peak below the temperature at which SM-1300 begins torecrystallize during cooling to the total area under the melting peak.FIG. 12 shows a heating curve and a cooling curve for a sample ofSM-1300 powder as shown by DSC measured at a scanning rate of 10°C./minute. One should note that in FIG. 12, the endothermic melting peakpoints downward and the exothermic cooling peak points upward, as thedisplay convention of the DSC apparatus used to generate FIG. 12 wasdirectly opposite from the convention of the DSC apparatus used togenerate FIGS. 11A and 11B. As shown by the cooling curve of FIG. 12,the sample of SM-1300 begins to recrystallize at approximately 99.5° C.The total area under the melting peak is measured as approximately 38.7Joules/gram. The area under the melting peak below this onset ofrecrystallization temperature, which is shaded and labeled with theletter B in FIG. 12, is measured as approximately 1.06 Joules/gram.Therefore, approximately 2.7% of the total area under the melting peakis below the onset of recrystallization temperature. Such little degreeof overlap between the melting peak and the recrystallization peakindicates that SM-1300 powder recrystallizes sufficiently slowly in aselective laser sintering process to eliminate in-build curl.

The preferred powders of Nylon 11, SC 912, and Affinity SM-1300 areexemplary and not limiting of this aspect of the present invention. Ingeneral, it is believed that certain nylons (other than Nylon 11),polyacetals, polypropylenes, polyethylenes, and ionomers exhibit similarmelting and recrystallization behavior in DSC scans and in the selectivelaser sintering process, and are therefore also preferred materialsaccording to this aspect of the invention. Other materials believed toexhibit similar melting and recrystallization behavior in DSC scans andin the selective laser sintering process, and which are therefore alsopreferred materials according to this aspect of the invention, includecertain copolymers of nylons, acetals, ethylenes, and propylenes (otherthan SC 912); certain branched versions of polyethylene andpolypropylene; and certain branched versions of polyethylene copolymers(other than SM-1300) and polypropylene copolymers. Copolymerization andbranching are modifications to the molecular structure of polymers thatcan be used to control the degree of crystallinity as well as the rateof recrystallization. All of the above-described materials preferablyexhibit an overlap between their melting and recrystallization peaks, asshown in DSC traces measured at a scanning rate of 10°-20° C./minute,ranging from 0° C. (no overlap) to no more than about 13° C. Morepreferably, all of the above-described materials exhibit such an overlapranging from 0° C. (no overlap) to no more than about 11° C. Inaddition, all of the above-described materials preferably exhibit apercentage ratio of the area under the melting peak below the onset ofrecrystallization temperature to the total area under the melting peak,as shown in DSC traces measured at a scanning rate of 10°-20° C./minute,ranging from about 0% (no overlap) to no more than about 6.2%. Morepreferably, the above-described materials exhibit such a percentageratio ranging from 0% (no overlap) to no more than about 2.7%.

It has been found that IP60, a particular high density polyethylenepowder sold by Dow Chemical, does not exhibit the above-describedmelting and recrystallization behavior in DSC scans, and therefore IP60powder does not work well in the selective laser sintering process. Morespecifically, FIG. 13 shows a heating curve and a cooling curve for asample of IP60 powder as shown by DSC measured at a scanning rate of 10°C./minute. As shown in FIG. 13, the IP60 powder exhibits an overlapbetween its melting and recrystallization peaks of approximately 24° C.,from about 97° C. to about 121° C. In addition, the cooling curve ofFIG. 13 shows that IP60 powder begins to recrystallize at approximately121° C. The total area under the melting peak is measured asapproximately 123.1 Joules/gram. The area under the melting peak belowthis onset of recrystallization temperature, which is shaded and labeledwith the letter C in FIG. 13, is measured as approximately 26.2Joules/gram. Therefore, approximately 21.3% of the total area under themelting peak of IP60 powder is below the onset of recrystallizationtemperature.

Similarly, it has also been found that Surlyn 8660, a particular ionomerpowder sold by Dupont, does not exhibit the above-described melting andrecrystallization behavior in DSC scans, and therefore Surlyn 8660powder does not work well in the selective laser sintering process. Morespecifically, FIG. 14 shows a heating curve and a cooling curve for asample of Surlyn 8660 powder as shown by DSC measured at a scanning rateof 10° C./minute. As shown in FIG. 14, the Surlyn 8660 powder exhibitsan overlap between its melting and recrystallization peaks ofapproximately 40° C., from about 45° C. to about 85° C. In addition, thecooling curve of FIG. 14 shows that Surlyn 8660 powder begins torecrystallize at approximately 85° C. The total area under the meltingpeak is measured as approximately 28.8 Joules/gram. The area under themelting peak below this onset of recrystallization temperature, which isshaded and labeled with the letter D in FIG. 14, is measured asapproximately 10.4 Joules/gram. Therefore, approximately 36.1% of thetotal area under the melting peak of Surlyn 8660 powder is below theonset of recrystallization temperature.

Accordingly, polymers that show little or no overlap between theirmelting and recrystallization peaks when scanned at typical DSC rates of10°-20° C./minute work best in a selective laser sintering process. Suchoverlap can be expressed in terms of °C. or in terms of a percentageratio of the area under the melting peak below the onset ofrecrystallization temperature to the total area under the melting peak.For example, wax, IP60 powder, and Surlyn 8660 powder are not suitablematerials by this test, while Nylon 11 powder, SC 912 powder, and SM1300powder are. Most suitable materials preferably have melting points below200° C. As noted above, suitable materials according to this aspect ofthe invention include Nylon 11 powder; SC 912 powder; SM-1300 powder;certain nylons (other than Nylon 11), polyacetals, polypropylenes,polyethylenes, and ionomers; certain copolymers of nylons, acetals,ethylenes, and propylenes (other than SC 912); certain branched versionsof polyethylene and polypropylene; and certain branched versions ofpolyethylene copolymers (other than SM-1300 ) and polypropylenecopolymers.

Having thus provided a general discussion, described the requirements ofa laser-sinterable powder in detail, and illustrated the invention withspecific examples of the best mode of making and using the powder, itwill be evident that the invention has provided an effective solution toa difficult problem. It is therefore to be understood that the claimsare not to be limited to a slavish duplication of the invention and noundue restrictions are to be imposed by reason of the specificembodiments illustrated and discussed.

                  TABLE 1                                                         ______________________________________                                                     Value                                                                                        Compression                                       Property       Last Sintered                                                                              Molded                                            ______________________________________                                        Thermal                                                                       Glass Transition (°C.)                                                 Melt (onset, °C.)                                                      Heat Distortion                                                               @ 264 psi (°C.)                                                                       46, 46       41, 41                                            @  66 psi (°C.)                                                                       163, 167     163, 159                                          TGA (onset of  not measured                                                   degradation)                                                                  Mechanical                                                                    Tensile                                                                       (5 mm/min crosshead)                                                          Modulus (psi) [σ]                                                                      201, 100, [10, 540]                                                                        207, 700 [11, 630]                                Elongation, ultimate (%)                                                                     28.0 [5.3]   201.6 [151]                                       Strength (psi) 6323 [157]   6315 [115]                                        Elongation, yield (%)                                                                        26.0 [3.3]   30.0 [1.3]                                        Energy to break (lb-in)                                                                      205 [53]     2, 149 [316]                                      Tensile                                                                       (50 mm/min crosshead)                                                         Modulus (psi)  221, 500 [28, 610]                                                                         227, 800 [18, 890]                                Elongation, ultimate (%)                                                                     27.0 [5.5]   271.8 [146.3]                                     Strength (psi) 6413 [130]   6200 [517]                                        Elongation, yield (%)                                                                        24.1 [32]    21.9 [93]                                         Energy to break (lb-in)                                                                      203 [43]     1, 995 [566]                                      Flexural                                                                      Modulus (psi)  146, 800 [4147]                                                                            176, 900 [4368]                                   Strength (psi) 7154 [159]   7044 [271]                                        Elongation, yield (%)                                                                        .091 [.002]  .065 [.002]                                       Izod Impact (notched)                                                         @  23° C. (ft-lb/in)                                                                  1.4 [.2]     1.89 [.24]                                        @ -40° C. (ft-lb/in)                                                                  1.03 [.2]                                                      Physical                                                                      Specific Gravity                                                                             1.0204 [.004]                                                                              1.0360 [.0004]                                    ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________           Feed Bed                                                                            Part Bed                                                                            Part Dens.                                                                          Full Dens.                                                                          Notch                                                                             HDT MAX STRESS                                                                            FLEX MOD                       Ex                                                                              Polymer                                                                            Temp. °C.                                                                    Temp. °C.                                                                    gm/cm.sup.3                                                                         gm/cm.sup.3                                                                         Impact                                                                            °C.                                                                        psi     psi                            __________________________________________________________________________    1 Nylon 6                                                                            140   180   0.958 1.04  1.5 175 11510   272100                         2 Nylon 11                                                                           135   165   0.919 0.987 1.67                                                                              166 8310    159900                         3 Nylon 12                                                                           75    160   0.90  1.01  0.39                                                                              163 8120    150750                         4 P'Acetal                                                                           130   150   1.283 1.41  0.72                                                                              149 9468    312400                         5 PBT  160   195   1.19  1.31  0.29                                                                              206 8270    299700                         __________________________________________________________________________     *(ft-lb/in): Izod impact, notched  measured at 23° C.             

We claim:
 1. A method of producing a three-dimensional object,comprising the steps of:applying a layer of a powder at a targetsurface, said powder comprised of a semi-crystalline organic polymer,said powder having a melting peak and a recrystallization peak, as shownin differential scanning calorimetry traces, which show little overlapwhen measured at a scanning rate of 10°-20° C./minute, and wherein saidpolymer is selected from the group consisting of nylon, polyacetal,polypropylene, polyethylene, and ionomers; directing energy at selectedlocations of said layer corresponding to the cross-section of the objectto be formed in said layer to sinter said powder thereat; and repeatingsaid applying and directing steps to form the object in layerwisefashion.
 2. The method of claim 1 further comprising the step ofremoving unsintered powder from said object.
 3. The method of claim 1wherein said overlap is no more than about 13° C.
 4. The method of claim1 wherein said overlap is no more than about 11° C.
 5. The method ofclaim 1 wherein said overlap, expressed as a percentage ratio of an areatinder said melting peak below an onset of recrystallization temperatureof said powder to a total area under said melting peak, is no more thanabout 6.2%.
 6. The method of claim 1 wherein said overlap, expressed asa percentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 2.7%.
 7. A method of producing athree-dimensional object, comprising the steps of:applying a layer of apowder at a target surface, said powder comprised of a semi-crystallineorganic polymer, said powder having a melting peak and arecrystallization peak, as shown in differential scanning calorimetrytraces, which show little overlap when measured at a scanning rate of10°-20° C./minute, and wherein said polymer is selected from the groupconsisting of copolymers of nylons, acetals, ethylenes, and propylenes;directing energy at selected locations of said layer corresponding tothe cross-section of the object to be formed in said layer to sintersaid powder thereat; and repeating said applying and directing steps toform the object in layerwise fashion.
 8. The method of claim 7 furthercomprising the step of removing unsintered powder from said object. 9.The method of claim 7 wherein said overlap is no more than about 13° C.10. The method of claim 9 wherein said polymer is SC
 912. 11. The methodof claim 7 wherein said overlap is no more than about 11° C.
 12. Themethod of claim 7 wherein said overlap, expressed as a percentage ratioof an area under said melting peak below an onset of recrystallizationtemperature of said powder to a total area under said melting peak, isno more than about 6.2%.
 13. The method of claim 12 wherein said polymeris SC
 912. 14. The method of claim 7 wherein said overlap, expressed asa percentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 2.7%.
 15. A method of producing athree-dimensional object, comprising the steps of:applying a layer of apowder at a target surface, said powder comprised of a semi-crystallineorganic polymer, said powder having a melting peak and arecrystallization peak, as shown in differential scanning calorimetrytraces, which show little overlap when measured at a scanning rate of10°-20° C./minute, and wherein said polymer is selected from the groupconsisting of branched polyethylene and branched polypropylene;directing energy at selected locations of said layer corresponding tothe cross-section of the object to be formed in said layer to sintersaid powder thereat; and repeating said applying and directing steps toform the object in layerwise fashion.
 16. The method of claim 15 furthercomprising the step of removing unsintered powder from said object. 17.The method of claim 15 wherein said overlap is no more than about 13° C.18. The method of claim 15 wherein said overlap is no more than about11° C.
 19. The method of claim 15 wherein said overlap, expressed as apercentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 6.2%.
 20. The method of claim 15wherein said overlap, expressed as a percentage ratio of an area undersaid melting peak below an onset of recrystallization temperature ofsaid powder to a total area under said melting peak, is no more thanabout 2.7%.
 21. A method of producing a three-dimensional object,comprising the steps of:applying a layer of a powder at a targetsurface, said powder comprised of a semi-crystalline organic polymer,said powder having a melting peak and a recrystallization peak, as shownin differential scanning calorimetry traces, which show little overlapwhen measured at a scanning rate of 10°-20° C./minute, and wherein saidpolymer is selected from the group consisting of branched polyethylenecopolymers and branched polypropylene copolymers; directing energy atselected locations of said layer corresponding to the cross-section ofthe object to be formed in said layer to sinter said powder thereat; andrepeating said applying and directing steps to form the object inlayerwise fashion.
 22. The method of claim 21 further comprising thestep of removing unsintered powder from said object.
 23. The method ofclaim 21 wherein said overlap is no more than about 13° C.
 24. Themethod of claim 21 wherein said overlap is no more than about 11° C. 25.The method of claim 24 wherein said polymer is Affinity SM-1300.
 26. Themethod of claim 21 wherein said overlap, expressed as a percentage ratioof an area under said melting peak below an onset of recrystallizationtemperature of said powder to a total area under said melting peak, isno more than about 6.2%.
 27. The method of claim 21 wherein saidoverlap, expressed as a percentage ratio of an area under said meltingpeak below an onset of recrystallization temperature of said powder to atotal area under said melting peak, is no more than about 2.7%.
 28. Themethod of claim 27 wherein said polymer is Affinity SM-1300.
 29. A lasersintered article, comprising a semi-crystalline organic polymer powderlaser sintered to form said article, said powder having a melting peakand a recrystallization peak, as shown in differential scanningcalorimetry traces, which show little overlap when measured at ascanning rate of 10°-20° C./minute, and wherein said polymer is selectedfrom the group consisting of nylon, polyacetal, polypropylene,polyethylene, and ionomers.
 30. The laser sintered article of claim 29wherein said article has a density of at least about 80% of the densityof a compression molded part of said powder.
 31. The laser sinteredarticle of claim 30 wherein said density is in the range from about 80%to about 95% of the density of a compression molded part of said powder.32. The laser sintered article of claim 29 wherein said overlap is nomore than about 13° C.
 33. The laser sintered article of claim 29wherein said overlap is no more than about 11° C.
 34. The laser sinteredarticle of claim 29 wherein said overlap, expressed as a percentageratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 6.2%.
 35. The laser sintered articleof claim 29 wherein said overlap, expressed as a percentage ratio of anarea under said melting peak below an onset of recrystallizationtemperature of said powder to a total area under said melting peak, isno more than about 2.7%.
 36. A laser sintered article, comprising asemi-crystalline organic polymer powder laser sintered to form saidarticle, said powder having a melting peak and a recrystallization peak,as shown in differential scanning calorimetry traces, which show littleoverlap when measured at a scanning rate of 10°-20° C./minute, andwherein said polymer is selected from the group consisting of copolymersof nylons, acetals, ethylenes, and propylenes.
 37. The laser sinteredarticle of claim 36 wherein said article has a density of at least about80% of the density of a compression molded part of said powder.
 38. Thelaser sintered article of claim 37 wherein said density is in the rangefrom about 80% to about 95% of the density of a compression molded partof said powder.
 39. The laser sintered article of claim 36 wherein saidoverlap is no more than about 13° C.
 40. The laser sintered article ofclaim 39 wherein said polymer is SC
 912. 41. The laser sintered articleof claim 36 wherein said overlap is no more than about 11° C.
 42. Thelaser sintered article of claim 36 wherein said overlap, expressed as apercentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 6.2%.
 43. The laser sintered articleof claim 42 wherein said polymer is SC
 912. 44. The laser sinteredarticle of claim 36 wherein said overlap, expressed as a percentageratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 2.7%.
 45. A laser sintered article,comprising a semi-crystalline organic polymer powder laser sintered toform said article, said powder having a melting peak and arecrystallization peak, as shown in differential scanning calorimetrytraces, which show little overlap when measured at a scanning rate of10°-20° C./minute, and wherein said polymer is selected from the groupconsisting of branched polyethylene and branched polypropylene.
 46. Thelaser sintered article of claim 45 wherein said article has a density ofat least about 80% of the density of a compression molded part of saidpowder.
 47. The laser sintered article of claim 46 wherein said densityis in the range from about 80% to about 95% of the density of acompression molded part of said powder.
 48. The laser sintered articleof claim 45 wherein said overlap is no more than about 13° C.
 49. Thelaser sintered article of claim 45 wherein said overlap is no more thanabout 11° C.
 50. The laser sintered article of claim 45 wherein saidoverlap, expressed as a percentage ratio of an area under said meltingpeak below an onset of recrystallization temperature of said powder to atotal area under said melting peak, is no more than about 6.2%.
 51. Thelaser sintered article of claim 45 wherein said overlap, expressed as apercentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 2.7%.
 52. A laser sintered article,comprising a semi-crystalline organic polymer powder laser sintered toform said article, said powder having a melting peak and arecrystallization peak, as shown in differential scanning calorimetrytraces, which show little overlap when measured at a scanning rate of10°-20° C./minute, and wherein said polymer is selected from the groupconsisting of branched polyethylene copolymers and branchedpolypropylene copolymers.
 53. The laser sintered article of claim 52wherein said article has a density of at least about 80% of the densityof a compression molded part of said powder.
 54. The laser sinteredarticle of claim 53 wherein said density is in the range from about 80%to about 95% of the density of a compression molded part of said powder.55. The laser sintered article of claim 52 wherein said overlap is nomore than about 13° C.
 56. The laser sintered article of claim 52wherein said overlap is no more than about 11° C.
 57. The laser sinteredarticle of claim 56 wherein said polymer is Affinity SM-1300.
 58. Thelaser sintered article of claim 52 wherein said overlap, expressed as apercentage ratio of an area under said melting peak below an onset ofrecrystallization temperature of said powder to a total area under saidmelting peak, is no more than about 6.2%.
 59. The laser sintered articleof claim 52 wherein said overlap, expressed as a percentage ratio of anarea under said melting peak below an onset of recrystallizationtemperature of said powder to a total area under said melting peak, isno more than about 2.7%.
 60. The laser sintered article of claim 59wherein said polymer is Affinity SM-1300.
 61. A three-dimensional objectformed by the method of claim
 2. 62. A three-dimensional object formedby the method of claim
 8. 63. A three-dimensional object formed by themethod of claim
 16. 64. A three-dimensional object formed by the methodof claim
 22. 65. A method of producing a three-dimensional object,comprising the steps of:applying a layer of a powder at a targetsurface, said powder comprised of a semi-crystalline organic polymer,said powder having a melting peak and a recrystallization peak, as shownin differential scanning calorimetry traces, which do not overlap whenmeasured at a scanning rate of 10°-20° C./minute, and wherein saidpolymer is selected from the group consisting of branched polyethylenecopolymers and branched polypropylene copolymers; directing energy atselected locations of said layer corresponding to the cross-section ofthe object to be formed in said layer to sinter said powder thereat; andrepeating said applying and directing steps to form the object inlayerwise fashion.
 66. The method of claim 65 further comprising thestep of removing unsintered powder from said object.
 67. A lasersintered article, comprising a semi-crystalline organic polymer powderlaser sintered to form said article, said powder having a melting peakand a recrystallization peak, as shown in differential scanningcalorimetry traces, which do not overlap when measured at a scanningrate of 10°-20° C./minute, and wherein said polymer is selected from thegroup consisting of branched polyethylene copolymers and branchedpolypropylene copolymers.
 68. The laser sintered article of claim 67wherein said article has a density of at least about 80% of the densityof a compression molded part of said powder.
 69. The laser sinteredarticle of claim 68 wherein said density is in the range from about 80%to about 95% of the density of a compression molded part of said powder.70. A three-dimensional object formed by the method of claim 66.